Because it offers a better experience. The cookies are not pointless to the experience and you need all of them to have the full experience. The legal definition about what cookies are needed does not match reality.
What parts of the experience do you feel are missing if you do not consent to tracking? I have seen one or two cases of malicious compliance where rejecting tracking results in no state being kept, including having rejected it. Keep in mind that the legal definition is based on things that would not be reasonably expected to be kept or distributed in order to provide the service that the user is getting, you can do basically everything except targeted ads or selling user data under that definition, even if people who want to do the above are trying to pretend otherwise.
Targeted ads are part of the experience. They directly affect user satisfaction of the product. Relevant ads can increase user engagement. You may find it strange, but people prefer products with relevant ads.
People prefer products without ads at all. Ads are noise. People's brains literally learn how to filter them out via banner blindness.
People always comment that the internet is "so much nicer" after I install uBlock Origin on their browsers. It's just better, they can't explain why. They don't need to. I know why.
The fact is nobody wants this crap. Ads are nothing but noise in our signal. They're spam. They're content we did not ask for, forced upon us without consent. They do not improve the "experience", at best its impact is minimized.
